The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An electrochemical capacitor in this case is a device for storing the electrical energy resulting from the separation of charged species and/or from redox reactions.
Recently, the interest in electrochemical capacitors has considerably increased as they are able to boost the power of systems. In the hybrid vehicles for example, the electrochemical capacitors are used to collect the braking energy and provide the power peaks during acceleration and slopes climbing. Applying electrochemical capacitor modules in hybrid vehicles allows a substantial amount of fuel to be saved. For full electric vehicles, capacitors can also contribute to increase the electrical yield.
The use of electrical double-layer capacitors where the charged species are separated at the electrode/electrolyte interface is known. In this case the electrical energy is stored in electrostatic form by charge separation.
The electrodes are usually made of activated carbons (hereinafter called “AC”). In the present disclosure, the wording “X/Y electrodes” means that the first electrode is made with material X and the second electrode is made with material Y.
There are different types of electrolyte. A first type is an organic electrolyte. Such an organic electrolyte means an electrolyte wherein the main component is not water and has no more than traces of water. Typically, the solvent of such organic electrolyte is acetonitrile or propylene carbonate. Organic electrolyte has the advantage of having a high maximum operating voltage U generally up to 2.7 V.
Therefore, organic electrolytes are usually preferred to obtain improved energy density, as shown in the following general formula:E=½CU2   (equation 1)                wherein the energy density (E) of an electrochemical capacitor is proportional to both the system's capacitance (C) and the square of voltage (U).        
However, the use of an organic electrolyte in an electrochemical capacitor implies higher costs, due to the cost of the electrolyte itself and to the fact that the modules forming the capacitor must be built in moisture-free atmosphere. Indeed, water limits the efficiency, the cycle life and the maximum operating voltage of such capacitor.
Moreover, organic solvents are environment unfriendly in comparison to the aqueous ones. The method for manipulating such organic electrolyte is costly for industry.
A second type of electrolyte is an aqueous electrolyte. An aqueous electrolyte means an electrolyte wherein the solvent is water.
The aqueous electrolyte has the advantage of providing pseudo-capacitance as well as electrical double-layer capacitance. With carbon electrodes, the pseudo-capacitive contribution is due to redox processes involving either surface functionalities or electrochemical hydrogen storage.
Aqueous electrolytes have also a higher conductivity than the organic ones. For example, the conductivity of a 1 mol·l−1 H2SO4 solution is about 1 S·cm−1, whereas the conductivity of a typical organic electrolyte is about 0.05 S·cm−1. The series resistance (RS) corresponding to the sum of all the resistances imposed by the elements forming the capacitor is therefore lower with an aqueous electrolyte than with an organic one. The contribution of the series resistance generally involves a higher power output in the presence of an aqueous electrolyte than in the presence of an organic electrolyte, as shown by the following formula:
                    P        =                              U            2                                4            *                          R              s                                                          (                  equation          ⁢                                          ⁢          2                )                            wherein the power (P) output by the capacitor is proportional to the square of voltage (U) but inversely proportional to the series resistance (RS).        
Nevertheless, the practical values for the maximum voltage obtained with aqueous electrolytes are typically lower than 1 V and thus lower than the maximum voltage obtained with organic electrolytes.
Recently, it has been built a capacitor with high maximum voltage values, namely 1.6 V, the capacitor comprising an aqueous electrolyte with H2SO4 and two different activated carbon, also called AC, electrodes in an asymmetric configuration. However, despite the advantage of such a capacitor, strong acidic medium remains difficult to use for industry due to the highly corrosive feature of the electrolyte. This feature raises a problem for finding cheap and efficient current collectors and cans.
Therefore, there is a need to provide a cheap and efficient capacitor, namely delivering high power and energy.
There is also a need to provide a capacitor which is environmentally friendly and easy to manipulate by a user.